Aromatic aldehydes which may be used in a variety of chemical reactions have, in the past, been prepared by various alternate reactions. For example, one method of preparing an aromatic aldehyde has been an air oxidation reaction in an oxygen-enriched environment utilizing relatively high temperatures and pressures in combination with a transition metal catalyst such as cupric bromide. Another method of effecting the preparation of aromatic aldehydes is by the chemical oxidation of the substrate using stoichiometric quantities of an oxidizing agent which is obtained by way of known electrochemical methods using concentrated sulfuric or perchloric acid, said reaction being effected at elevated temperatures. Yet another basic synthetic reaction for obtaining aromatic aldehydes is the chemical oxidation of the substrate using stoichiometric quantities of electrochemically generated oxidants such as salts of cobalt, manganese, or chromium in their highest valence state in a strongly acidic media at elevated temperatures. Reduced oxidant is then recycled, purified and electrolytically reoxidized back to its active state.
The inherent drawback in the last named reaction involves the reoxidation and recycling of the oxidant by electrochemical methods. Heretofore all of the methods which have been employed in this area have oxidized the transition metal to its higher valence state prior to combination of the same with the organic substrate in a conventional chemical reactor. In essence, this comprises a two-step reaction which requires both an electrochemical reactor and a chemical reactor. In addition, the aforementioned processes have utilized relatively concentrated acids such as from 40% to 70% concentration of sulfuric acid or perchloric acid, thus making the selectivities of these processes for activated alkyl aromatic systems less than desirable. The undesirability of these processes results from the tendency of the alkyl aromatic systems towards sulfonation or by-product formation.
Prior work in the oxidation of aromatic compounds has been shown in the U.S. Pat. No. 4,046,652. However, this patent describes the oxidation of an aromatic nucleus in an electrochemical reaction to form p-benzoquinone diketals. The electrolyte which is used in this electrochemical reaction comprises methyl alcohol containing a conducting salt, preferably an ammonium or alkali metal salt of an acid such as hydrofluoric acid, perchloric acid, nitric acid, etc. Likewise, U.S. Pat. No. 4,148,696 also relates to electrochemical oxidation reactions involving aromatic compounds. However, this patent involves an anodic acyloxylation involving the use of a salt of a fatty acid. Another patent, namely U.S. Pat. No. 4,101,392 discloses a process for the electrolytic oxidation of aromatic compounds. However, this patent is concerned with a process for the methyl-methyl coupling of hydroxy aromatic compounds, which process is in contradistinction to the process of the present invention, hereinafter set forth in greater detail, which is concerned with the oxidation of the methyl substituent of a methyl-substituted aromatic compound. An article which appeared in the Bulletin of the Chemical Society of Japan, volume 37, number 11, has disclosed an electrochemical process for the methoxylation of aromatic compounds. This anodic oxidation was effected by treating an aromatic compound such as tetralin, indane, or diphenylmethane, to afford a methoxy-substituted aromatic compound. However, this process is dissimilar from the process of the present invention in which the methyl-substituent on the ring of an aromatic compound is converted to an aldehyde.
As will hereinafter be set forth in greater detail, it has now been discovered that the oxidation of an alkyl aromatic compound in an electrochemical reaction may be effected in the presence of a nucleophile to form an acetal which is then converted to the desired aldehyde.